The building of underground structures, in particular passages under operational traffic lanes, roads or railways, presents problems owing to the fact that the traffic must be able to continue while the work is performed. This building work requires a great number of precautions and in particular forces the traffic, particularly railway traffic, to slow down.
It has already been proposed, in the Swiss Patent Specification No. 442404, to form a hollow reinforced concrete structure which has a parallelepipedal shape and is axially open and the cross-section of which corresponds to that of the underground passage. The front edge of this structure is formed with a cutting edge such that it is able to be driven into the ground over a certain length by means of jacks. The part of the ground inside this hollow structure is subsequently excavated. When the excavation process has reached the front end of the reinforced concrete structure it is again driven in over a certain length, then the enclosed material is excavated, and so on until the passage has been completely cut.
Although this solution is intended to overcome the problems which are inherent in the cutting of an underground passage below an operational traffic lane by avoiding the slowing-down of traffic while work is in progress, the implementation thereof requires a considerable amount of equipment, in particular the installation of high-power jacks as well as structures which are able to support these jacks. The continuous advancing of the reinforced concrete structure also presents problems with respect to the length of the jacks since the hollow structure advances while the supporting constructions remain stationary. In addition, the required pressure is very great in view of the friction surface which increases as this structure is driven in. Consequently, this solution presents relatively complex problems with respect to implementation which make it expensive owing to the additional work necessitated by the structures which support the jacks. Although it cannot be denied that a solution of this type enables the problem of cutting a passage under an operational traffic lane without slowing down the traffic to a great extent to be overcome, this involves considerable practical difficulties and an increase in the building costs.
It has already been proposed in U.S. Pat. No. 4,009,579 to drive in adjacent tubes which are disposed essentially horizontally in the ground and are intended to extend longitudinally at the location where the upper part of an underground structure will be defined. Subsequently excavation is performed below the zone which is covered by tubes and, as this work progresses, props are arranged in the form of frames which are formed by means of reinforced concrete girders directed along the successive cross-sections of this construction. The sequence of these frames adjacent to one another thus forms a continuous wall. These frames are interconnected by mortar injected into the space separating these frames, on the one hand, from the tubes and, on the other hand, from the surface excavated in the ground.
The substantially prefabricated building of this structure is relatively slow. The positioning of the adjacent frames is complicated owing to the fact that the excavation of the ground is not precisely defined and thus requires each frame to be propped up. The filling with mortar of the spaces occurring between the excavated volume and the frames involves additional consumption of cement while the connection of the frames by means of mortar imparts relatively low strength to the construction such that the frames at the end of the passage have to be axially retained by a support structure.
Thus the above solution is neither technically nor economically satisfactory.
The German Offenlegungschrift No. 1759309 likewise relates to the building of an underground construction which is carried out by means of tubes arranged adjacent one another in the section which is to form the ceiling of the construction and connected to one another by tension elements anchored to these tubes. These tubes are positioned by being driven into the ground, after which a volume is excavated below these tubes. As excavation progresses these tubes are supported by means of props in the form of frames arranged along the cross-section of the construction. These tubes are filled with prestressed concrete and the empty spaces remaining between them are filled with sprayed concrete. The interior of the construction is then provided with conventional masonry. This document does not specify of what the load-bearing structure consists or the nature of the internal masonry, whether it is simply a covering or a reinforced wall. Nevertheless it does not seem possible that the load-bearing structure only comprises tubes filled with prestressed concrete, owing to the lack of cohesion of a construction of this type, each girder being independent of the adjacent girder, and on account of the absence of lateral bearing walls. The masonry works necessary for finishing and the nature of which is not specified should be great. Finally, the fixing of the tension elements anchored to the tubes in order to connect them to one another presupposes that the tubes have a diameter sufficient to enable a man to pass therethrough in order to carry out this work. As these tubes are subsequently filled with concrete the volume of concrete required is considerable. Furthermore, these very large concrete cylinders rest on the masonry structure which is to be the support and therefore require large-scale casing and reinforcing works. This solution is technically and economically ill-suited to the problem of the building of an underground construction.
It is proposed in Belgian Pat. No. 872754 to drive tubes into the ground in order to form a vault resting on walls at its two lateral ends, prestressed concrete girders being formed transversely in order to support these tubes filled with concrete, after which the construction is excavated. This method presupposes that the tubes have a diameter sufficient to enable a man to move about therein, all the masonry work, including the walls supporting the vault, being finished when excavation is peformed.
These solutions often only provide a relative simplification of the building of an underground construction, require the assembly and dismantling of formwork or presuppose that the tubes have a large diameter, and are ill-suited to the building of passages under roads, for example. The limits of the volume to be excavated are only defined in the above-mentioned Belgian patent specification and in this case the concrete walls supporting the vault have to be formed before excavation, which, apart from the tubes with a large diameter, requires galleries to be cut which presents problems with respect to the removal to the exterior of the materials produced by ecavation of the galleries and the volumes defining these bearing walls. Evidently, a method of this type is not suitable for the building of constructions such as passages under roads.